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Topic: Nucleophilic Substitution  (Read 12296 times)

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Offline deutdeut

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Nucleophilic Substitution
« on: February 09, 2007, 04:53:29 AM »
For a secondary halide, when a weak base is used, SN2 mechanism is more favoured. Then would SN1 mechanism occur to a lesser extent at the same time? How about when NAOH solution is used. Is it still SN2 dominates over SN1?

Offline Dan

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Re: Nucleophilic Substitution
« Reply #1 on: February 09, 2007, 08:35:11 AM »
What's your nucleophile? is it the base? Can you post an example?

Rate of Sn1 vs Sn2 on a sec halide depends on the strength of your nuclephile (note that strong nucleophile is not necessarily a strong base), solvent used and the identity of the halide. Also note that a strong base may cause elimination rather than substitution.
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Offline deutdeut

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Re: Nucleophilic Substitution
« Reply #2 on: February 09, 2007, 09:36:47 PM »
I know that secondary halides can undergo both SN1  and SN2 mechanisms. Consider they react with weak bases, like iodide and cyanide ions, my book says that they mainly undergo SN2 mechanisms. I want to know does it mean that actually BOTH mechanisms take place in these reactions but SN2 predominates , say, constituting about 90%? And we would expect to get a mixture of products from both mechanisms?

Offline enahs

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Re: Nucleophilic Substitution
« Reply #3 on: February 09, 2007, 10:05:52 PM »
In order for a SN1 mechanism to take place, it has to form a stable carbocation. This is generally only possible to any real significant extent for tertiary substrates. There is however another mechanism you will cover called Elimination. Both E1 and E2, and for secondary substrates E2 can compete with SN2; and this is probably what your book is referring to.

Offline deutdeut

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Re: Nucleophilic Substitution
« Reply #4 on: February 09, 2007, 10:23:39 PM »
So, in secondary halides, what mechanisms are involved in reacting with weak bases which are weak nucleophiles, like ethanol?

Offline enahs

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Re: Nucleophilic Substitution
« Reply #5 on: February 09, 2007, 10:33:05 PM »
So, in secondary halides, what mechanisms are involved in reacting with weak bases which are weak nucleophiles, like ethanol?

Why do you say ethanol is a weak base?

But in any case, a secondary substrate with a weak base gives mainly SN2.
But since ethanol is a strong base, it will give a E2 reaction for a secondary substrate.

Offline english

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Re: Nucleophilic Substitution
« Reply #6 on: February 09, 2007, 10:33:27 PM »
A poor nucleophile, ignoring the solvent used, will favor an SN1 reaction for a 2° alkyl halide.

A good nucleophile favors the SN2 reaction.

Offline english

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Re: Nucleophilic Substitution
« Reply #7 on: February 09, 2007, 10:35:43 PM »
So, in secondary halides, what mechanisms are involved in reacting with weak bases which are weak nucleophiles, like ethanol?

Why do you say ethanol is a weak base?

But in any case, a secondary substrate with a weak base gives mainly SN2.
But since ethanol is a strong base, it will give a E2 reaction for a secondary substrate.

Ethanol is not very basic in character...at all.  Alcohols direct E1 mechanisms, not E2. 

Offline deutdeut

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Re: Nucleophilic Substitution
« Reply #8 on: February 09, 2007, 10:40:28 PM »
That means , secondary halides when reacting with a weak base, SN2 competes with elimination reactions?

Offline enahs

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Re: Nucleophilic Substitution
« Reply #9 on: February 09, 2007, 10:45:24 PM »
Ethanol is not very basic in character...at all.  Alcohols direct E1 mechanisms, not E2. 

Ethanol is/can be considered a strong base; and as far as I know most people do. It in fact can be a stronger base then hydroxide; that is ethanol -> ethoxide ion, a strong base.

Secondary substrates, such as in this case he was asking about, do not typically undergo E1 mechanism; because they can not form stable carbocations.

Ok, keep in mind he is just learning this, and has not yet covered rearrangements yet, so you must answer base your reply s off that. :)

Offline english

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Re: Nucleophilic Substitution
« Reply #10 on: February 09, 2007, 10:47:09 PM »
E1 and SN1 reactions use poor nucleophiles.  SN2 and E2 mechanisms use strong nucleophiles.

An example of a poor nucleophile is an alcohol.

An example of a good nucleophile is sodium methoxide, NaCH3O-.

Offline english

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Re: Nucleophilic Substitution
« Reply #11 on: February 09, 2007, 10:50:32 PM »
Ethanol is not very basic in character...at all.  Alcohols direct E1 mechanisms, not E2. 

Ethanol is/can be considered a strong base; and as far as I know most people do. It in fact can be a stronger base then hydroxide; that is ethanol -> ethoxide ion, a strong base.

Secondary substrates, such as in this case he was asking about, do not typically undergo E1 mechanism; because they can not form stable carbocations.

Ok, keep in mind he is just learning this, and has not yet covered rearrangements yet, so you must answer base your reply s off that. :)

I disagree.  If you have a rapid equilibirum mixture of ehtanol and ethoxide, then ethoxide will be your good nucleophile...not ethanol. 

Ethanol is a sufficiently good nucleophile for an SN1 mechanism...maybe I am not understanding what you're referring to.

Offline deutdeut

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Re: Nucleophilic Substitution
« Reply #12 on: February 09, 2007, 10:50:59 PM »
So, if determining whether SN1 or SN2 compete with elimination, we just need to consider nucleophilicity but need not worry about the basicity of the nucleophiles?

Offline english

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Re: Nucleophilic Substitution
« Reply #13 on: February 09, 2007, 10:56:45 PM »
So, if determining whether SN1 or SN2 compete with elimination, we just need to consider nucleophilicity but need not worry about the basicity of the nucleophiles?

What characteristics do nucleophiles share?  Not all nucleophiles are bases, but they all have something that allows them to do their job.

Offline enahs

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Re: Nucleophilic Substitution
« Reply #14 on: February 09, 2007, 10:58:18 PM »
I disagree.  If you have a rapid equilibirum mixute of ehtanol and ethoxide, then ethoxide will be your good nucleophile...not ethanol. 

Ethanol is a sufficiently good nucleophile for an SN1 mechanism...maybe I am not understanding what you're referring to.

He specifically said a secondary substrate, and relating it to what type (elimination or substitution).

For a secondary substrate, ruling out the possibility of rearrangement, with a weak base you will get predominately a SN2 reaction and with a strong base predominately a E2 reaction. That is not to say you will not get others, but in general based on the concepts he is learning; those should be the predominate ones he should expect.


And I should clarify for him, yes ethanol can be weak or strong, it depends on the solvent and if it forms the ethoxide ion.

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